Finished iron-chromium alloy article and method of making the same



Patented May 24, 1932 UNITED STATES PATENT omca S. GEORGE, 0] WIGUA, NEW YORK, ABSIGIOB nae-r IE'IAI-LUBGIGAL COIPANY, A. CORPORATION 01 WEST VIRGINIA mow-mom ALLOY ABTIGLE AID IE'IBOD 0! name sum I0 Drawing. Application Med December 21, 1928,

My inventionrelates to finished articles consisting of iron-chromium alloys contaming 8% to chromium, andv especially to rustless iron and stainless steel articles which contain less than 3% carbon and to methods of roducing the same.

icles consisting of these iron-chromium allo s are shaped by hot rolling or for ing methods. Heretofore, the articles have 11 box annealed at 800 to 1000 C. to bring about structural changes in the alloy and to produce ductility. The rolled or forged SI', ticles are covered with an oxide scale and the annealing is done with the oxide present on the surface of the metal. The surface oxide on these box annealed metals is extremel resistant to pickling acids and on some 0 the metals it is practically immune to the action of acid. The removal of the surface oxide on such annealed articles has required a rolonged treatment in an acid bath, a grin ng operation, or sand blasting. These cleamng methods remove a considerable amount of' metal and destroy the surface finish and they are laborious and costly. It has not been helpful to remove the oxide before the metal is box annealed because a similar acid resistant oxide is roduced when clean metal is box anneale and the same destructive methods had to be used to clean it.

I have discovered new methods of treating such alloys which avoid these difliculties and reduce the cost of production. Theoxides which occur on these alloys after theyare hot rolled ma be removed in acid pickling baths. I pre er to remove the oxides in a dilute acid bath consisting of a mixture of nitric and hydrochloric acids but other acids may used. For example, the hot rolled scale on a rustless iron containing 18% chromium may be removed in a bath consisting of 10 grams per liter hydrochloric acid and grams per liter'nitric acid at about C. without attacking the metal. This bath will not attack the metal and destroy the hot rolled finish but will readil loosen the scale so that it will fall off. higher concentration of hydrochloric acid may be used for alloys containing a higher content of chromium and more dilute acids Serial Io. 327,751. Renewed Icvembe'r 23, 1981.

are suitable for alloys of lower chromium content. These acids cause the hot rolled scales to peel or slough ofi practically undissolved. J

After the oxide is removed the article is box annealed in an atmosphere which is preferably non-om'dizing or reducing and which at all events is not strongly oxidizing. A coating of oxide so heavy that the finish will be destroyed or'the metal will be unevenl or diversely oxidized is avoided. IA reducng atmosphere may be produced by displacing the air which is initially entrapped in the box with any reducing gas which willvnot deposit carbon when heated to 800 to 1000" C. in contact with the metal. Box annealing the'metal in contact with carbon in any form is to be avoided as the carbon forms a coatmg on these iron-chromium alloys which is difiicult to remove. I have used a small quantity of wood charcoal in the annealing box out of contact with the metal to produce the desired atmosphere and'carefully luted the box to prevent oxidizing gases from entering. For thls purpose, I prefer to use lump wood charcoal. The charcoal combines with practically all of the oxygen entrapped in the box and prevents the formation of more than an extremely thin film of oxide on the metal. The metal is heated to about 800 to 1000 C. for a time suflicient to bring about ductility and structural changes according to the annealing practice which is well understood in the art.

The annealed article has 'a thin coating of oxide which resembles the oxide surface known as temper color. For the purposes of this invention, this thin coating will be re: ferred to herein as an iridescent coating.

The coating is so thin that the article can be considered as finished at this stage but if desired a novel method'of cleaning may be used to remove the iridescent coating. The iridescent coating is so resistant to acid that it cannot be removed in an acid bath without defrom metals, Serial No. 327, 56, filed Decemher 21, 1928. In accordance with this method the article may be treated in a 15% to 30% sodium hydroxide solution to which chlorine has been added until the solution is clouded with liberated oxygen. Merely dipping the article in this solution is usually sufficient but a longer treatment in the alkali solution is not harmful. The alkali changes the character of the scale so that acids will remove it. I prefer to use an acid bath which will not materially attack the metal. Dipping the alkali treated article into a bath like the one used to remove the hot rolled scale will ordinarily remove the iridescent coating in a few seconds without attacking the metal or materially changing the character of the surface given to it in hot working.

My invention provides a new annealed article of iron-chromium alloy with a finished surface of an iridescent oxide, which is so thin that the utility of the article is not impaired for many purposes. An article with an oxide-free surface is provided by less costly methods than those heretofore used and extremely small amounts of metal and pickling materials are consumed.

Since various modifications of the process may be made without departing from my invention, I do not wish to limit my invention except by the prior art and by the invention as defined in the annexed claims.

I claim as my invention:

1. An annealed article consisting of an iron-chromium alloy containing 8% to 40% chromium having a hot rolled finish and a thin iridescent oxide coating.

2. An annealed article Consisting of an iron-chromium alloy containing 8% to 20% chromium having a hot rolled finish and a thin iridescent oxide coating.

3. An annealed iron-chromium alloy containing 8% to 40% chromium and carbon not in excess of, 3% and having an un olished unetched hot rolled surface finish ree from surface oxides.

4. The method of producing a finished article of iron-chromium alloy containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove only the surface oxides and then box annealing in the presence of wood charcoal which is out of contact with the metal.

5. The method of producing a finished iron-chromium alloy article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove only the surface oxides and then box annealing in a non-carburizing atmosphere which is not strongly oxidizing.

6. The method of producing a finished iron-chromium alloy article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove only the surface oxides and then box annealing at800 to 1000f C. in the presence of solid carbon under such conditions that the carbon is out of contact with the article whereby to yield an atmosphere which is non-carburizing and not strongly oxidizing.

7. The method of producing a finished iron-chromium article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove only the surface oxides, heating to 800 to 1000 C. in a closed container in the presence 0 solid carbon under such conditions that the carbon is out of contact with the metal whereby to yield an atmosphere which is non-carburizing and not strongly oxidizing, immersing in an alkali bath and then in an acid bath which will not attack the metal.

8. The method of producing a finished iron-chromium article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove only the surface oxides, heating to 800 to 1000 C. in a closed container in the presence of solid carbon under such conditions that the carbon is out of contact with the metal whereby to yield an atmosphere which is non-carburizing and not strongly oxidizing, immersing in an alkali bath containing an oxidizing agent and then in an acid bath.

9. The method of producing a finished iron-chromium alloy article containing 8% to 40% chromium which comprises treating the hot rolled article in a bath at about C. containing about 10 grams per liter hydrochloric acid and 50 grams per liter nitric acid, annealing in a closed container in the presence of solid carbon under such conditions that the carbon is out of contact with the metal, treating in a solution containing about 20% sodium hydroxide and a small amount of chlorine and then in said acid bath.

10. The method of producing a finished iron-chromium article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath to remove surface oxides so that the surface finish will not be impaired, heating to 800 to 1000 C. in a closed container in the presence of wood charcoal under such conditions that the charcoal is out of contact with the metal, treating in an alkali bath and then in an acid bath.

11. The method of producing a finished iron-chromium article containing 8% to 40% chromium which comprises treating the hot rolled article in an acid bath so as to remove surface oxides without impairing the surface finish, annealing in a closed container in the presence of solid carbon under such conditions that the carbon is out of contact with the metal, treating in a bath containing about 20% sodium hydroxide to which chlorine has been added and then in an acid bath containing about 10 grams per liter hydrochloric acid and 50 acid. I

12. The method of producing a finished iron-chromium article containing 8% to 40% chromium which com rises treatin the hot rolled article in an aci bath contaimng about 7 10 grams per liter hydrochloric acid and 50 grams per liter nitric acid, annealing in a closed container in the presence of wood charcoal under such conditions that the charcoal is not in contact with the metal, treating in a bath containing about sodiumhydroxide to which chlorine has been added and then in the said acid bath.

In testimony whereof I afiix my si ature.

HARRY S. GEO GE.

grams per liter nitric- 

